Data that is to be transmitted from a transmitter to a receiver in a communication system is in general coded and modulated before it is transmitted over a link between the transmitter and the receiver.
In wireless communication systems, the link between the transmitter and the receiver is a wireless link. In this document this link is hereafter called a channel. The channel uses a frequency interval for the transmission of data. The channel may be divided into subchannels, each subchannel using a fraction of the frequency interval for transmitting data. Data is encoded and modulated before it is transmitted over the channel. In communication systems in general, one modulation and coding scheme is used for coding and modulating all data to be transmitted over a link from a transmitter to a receiver within a certain time period called e.g. transmission time interval (TTI) or time frame.
In wireless systems, such as GSM, W-CDMA, CDMA2000, WIMAX etc., the quality of the channel over the frequency interval could vary much more than the quality of the link in fixed communication systems. As a result, a coded data word will typically be transmitted over subchannels experiencing different quality. The quality of a channel could be estimated by measuring e.g. signal to interference and noise ratio (SINR) over the frequency interval. For wideband wireless systems, the channel covers a broad frequency range, i.e. the frequency interval is broad. Therefore, the channel quality variation over the whole frequency interval within a TTI is normally higher than for narrowband systems.
For this reason, channel adaptation is widely used, whereby modulation and coding scheme is dynamically selected for the channel according to channel condition, e.g. the quality of the channel. This is done for example in the wireless technologies High Speed Downlink Packet Access (HSDPA) or CDMA Evolution Data Only (CDMA EV-DO). CDMA EV-DO is the 1st step of CDMA2000 evolution with some channels for data-service-only, in addition to the traditional channels that support speech services only. In existing wireless communication systems, channel adaptation is traditionally designed such that a single modulation mode and a single code rate is used per TTI or time frame for the channel. I.e. the same code rate and the same modulation mode is used per TTI to code and modulate data onto all subchannels used for the transmission.
When transmitting data over a channel, there is an interest to achieve a high throughput over a channel for a certain transmit power level.
Simulations performed by the inventors show that there are inherent losses in throughput for a channel experiencing high channel quality variation. According to the simulation results, there is a need for a lower uniform coding rate if a channel experiences high channel quality variation than if the channel experiences low channel quality variation. Or seen in another way, the larger the quality variation, the higher transmit power is needed to achieve the same target block error rate (BLER). These simulation results are shown in the diagram of FIG. 1. In the simulations, each subchannel of one channel could take either a first or a second Signal to Noise Ratio (SNR) state. The example figures are given as SNR since they are link simulation results or single-cell simulation results, where SNR equals SINR. The curves in the diagram show required average SNR and corresponding required code rate for achieving a certain BLER. Each curve corresponds to a certain offset in dB between the two SNR states. As can be seen in FIG. 1, for a certain average SNR, there is a need for a lower coding rate to achieve the same BLER when the two SNR states are widely separated compared to when they are more close to each other. For example, when the two states have the same level, i.e. are offset with 0 dB (the 0 dB curve), the coding rate could be 0.9 for an average SNR level of 8 dB, but when the two states are offset with 30 dB, (see the 30 dB curves, i.e. the curve furthest up in the diagram) but still an average SNR level of 8 dB is used, the coding rate has to be 0.43. Inversely, when the two states are offset with 0 dB, and a coding rate of 0.43 is used, an average SNR level of 0 dB could be used, compared to an average SNR level of 8 dB for the case of 30 dB offset and with the same coding rate of 0.43 to achieve the same BLER.
To further give some hints of the influence of the channel quality variance on the channel throughput, further simulations resulted in the four diagrams of FIG. 2. The diagrams of FIG. 2 show the normalized throughputs for different channel quality conditions. In FIG. 2, the SNR is assumed to be log-Normal distributed. In the upper left diagram, the standard deviance for the SNR over the channel is 5 dB, in the upper right diagram it is 10 dB, in the lower left diagram 15 dB and in the lower right diagram it is 20 dB. The continuous line in each diagram shows the throughput for perfect link adaptation, i.e. when each subchannel has been allocated an individual modulation and coding scheme (MCS), based on its SNR level (or state). The broken line in each diagram shows the throughput for single MCS selection per frame, i.e. when one modulation and coding scheme has been selected for all subchannels, regardless of SNR level. In the simulations the same code block size has been used for both single MCS and perfect link adaptation, such that the single MCS and perfect link adaptation methods can be compared, irrespective of block size. Also, no regards has been taken to possible differences in size of necessary overhead information. The modulation in FIG. 2 has been selected among the modulation schemes BPSK, QPSK, 16QAM, 64QAM, for both the single MCS case and the perfect link adaptation case. Since two bits per OFDM symbol have to be used to communicate to the receiver which modulation scheme has been used, the normalized throughput increases from 0 to a maximum of 6 bits/symbol with the increasing of the expectation of SNR in dB of the multi-state channel.
FIG. 2 shows that there is a gap in throughput between the case when the same MCS is used for the whole channel, and the case when an individual MCS is selected for each subchannel, when the assumption of same block size and same amount of overhead information is used. FIG. 2 also shows that the gap increases the larger the channel variation. I.e. the simulations have shown that there is an obvious space for improvement of the link adaptation when channel varies significantly during one TTI/frame. It may be noted that the performance difference shown in FIG. 2 pertains to practical codes and code block sizes. In the ideal case of perfect codes and infinitely large code blocks, the performance difference would be expected to vanish when the same block size is used.
As shown above, the drawbacks of the single MCS selection per frame or TTI include:                Throughput loss in a channel with serious quality variation since the imperative uniform code rate and modulation mode limit the throughput for data sent over subchannels with high quality (for good channel states).        Channel estimation is more prone to errors for channel states with low quality. With bad channel estimates in the uniform MCS link adaptation, the inclusion of low-quality states effectively subtracts energy from better-quality states.        
In FIGS. 1 and 2 it has been shown that the throughput will be higher if the modulation and coding scheme would be adapted to the channel quality level, or the SNR level, if the same block sizes are assumed. Then it may be assumed that the solution would be to use the perfect link adaptation method, where MCS adaptation is done for each subchannel, depending on its quality state. Although, there are other drawbacks with the perfect link adaptation method:                The performance, or actual throughput, is limited since the code blocks have to be made small because each fraction of the codeword has its own MCS. FIG. 2 was a simulation where the same code block size was assumed in the two methods, to be able to compare the throughput of the two methods, regardless of code block size;        Too much overhead is needed to inform the receiver of the MCS used in each code block.        
In total, when code block size and size of overhead information is included, for the perfect link adaptation method the actual throughput would be poor.
There are also proposals of mixed-modulation per TTI/frame for link adaptation recently. Such a proposal is described in a standardisation contribution presented at 3GPP TSG RAN WG1 #42 on LTE R1-050942 in London, UK, Aug. 29-Sep. 2, 2005, by NTT DoCoMo, NEC, SHARP, with the title: AMC and HARQ Using Frequency Domain Channel-dependent Scheduling in MIMO Channel Transmission. In this document, modulation adaptation is done for each chunk or subchannel depending on its state or quality level. This scheme causes some performance improvements since higher-order modulation mode is used for subchannels experiencing high SINR, and lower-order modulation mode is used for subchannels experiencing low SINR. However, mixed-modulation scheme requires significant extra signaling cost compared to a single MCS case, since it is necessary to inform the receiver for each transmitted coded and modulated data word fraction which modulation scheme that has been used. If there are four different modulation schemes to choose from, each coded and modulated data word fraction would need to include two bits stating the used modulation scheme.
As shown above, there is a need for achieving high throughput over a channel, and there is space for improving the throughput for a channel experiencing high quality variations.